The Last Phytosaur

Leaving the field is always bittersweet. After a week scrambling over rocks in search of fossils, a warm shower and cozy bed are the ultimate luxuries. But no matter what an expedition was like, I always feel like I’m leaving too soon. If I went prospecting and failed, I wonder what I would have found if I had spent one more day among the outcrops. If I found a new site or uncovered new bones in a quarry, I want to stay and dig in further. There’s always an excuse to spend more time in the past.

Paleontological diversions on the way home ease the pain. And as I was getting ready to leave a field camp in a remote stretch of eastern Utah last week, paleontologist Andrew Milner recommended a little sidetrip I had never heard of before – The Last Phytosaur.

You could say that modern crocodiles are mimicking phytosaurs. Very few people outside paleontological disciplines would understand you, of course, but the connection would still be on the mark. Back in the Triassic – the first act of Earth’s Mesozoic epic – phytosaurs were armored, long-jawed, snaggletoothed ambush predators that proliferated through freshwater lakes and rivers. They perfected the semiaquatic lifestyle now enjoyed by gharials, alligators, and crocodiles, albeit with a few differences. Phytosaurs did not have a bony palate in their mouths like crocodylians do, for example, and the nostrils of phytosaurs were pushed back far on their snouts, sometimes as far back as their eyes.

The phytosaurs are long gone. They did not survive the mass extinction that dramatically altered life on Earth at the end of the Triassic, about 201 million years ago. And while we’ll probably never discover the tragic phytosaur that marked the very end of its lineage, The Last Phytosaur is a fossilized proxy for that poor creature.

Paleontologists Michael Morales and Sidney Ash named the specimen, albeit with a question mark, in a short 1993 missive in the New Mexico Museum of Natural History and Science Bulletin. The fossil was found on the underside of a sandstone ledge, exposed by the serendipity of erosion.

When I walked out to the site with paleontologist Adam Huttenlocker, though, I wasn’t exactly sure what I was looking for. Stumbling through the rock field, dessicated tumbleweeds scratching my legs, I searched the overhangs for some sign of phytosaur bone. Nothing. The fossil was supposed to be incredibly obvious. How could I miss it?

Huttenlocker and I decided to try one more ledge before hitting the road. And sure enough, there it was. Not a skeleton, but a natural mold of the phytosaur’s skull, impressed into the orange sandstone.

Aside from searching the wrong outcrop, it took so long to find the phytosaur because I was looking too low. Phytosaurs were Triassic creatures, after all, so I was stubbornly scanning the Late Triassic Chinle Formation. That’s the formation in which I found phytosaur bones and teeth at sites elsewhere in the west. But The Last Phytosaur is in the overlying Wingate Sandstone. I didn’t expect to see it there.

For a long time the Wingate sandstone was thought to represent the earliest part of the Jurassic. If this were the case, then The Last Phytosaur would be a straggler that expired after the worst of the extinction pulse had passed. But it turns out that the bottom of the Wingate sandstone documents the end of the Triassic, shading into the earliest part of the Jurassic towards the top.

And, as I later learned once I got to read Morales and Ash’s paper, The Last Phytosaur wasn’t alone. Bone jutting from the same sandstone ledge – spotted by Huttenlocker during our visit – is part of a second skull that is otherwise ensconced in the Wingate sandstone. It may be impossible to remove.

What species these phytosaurs belong to is difficult to tell, but Morales and Ash tried to narrow down the possibilities a bit. Taking a latex peel of the skull mold, they created a plaster cast from the impression. The closest match looked to be an animal now called Machaeroprosopus, a phytosaur with a deep skull and nostril opening up near the eyes.

Peering up at the overhang, I couldn’t narrow down the identification any further than “phytosaur.” But despite my ignorance of the fossil’s taxonomic assignment, I was taken by what the skull impression represented.

While the Triassic is often called the “Dawn of the Dinosaurs”, the truth is that dinosaurs weren’t major players for most of the period. They were typically small and rare, eking out a living alongside a menagerie of even stranger creatures, the trap-jawed phytosaurs among them. When these creatures – the highly-armored aetosaurs, formidable rauisuchians, and their ilk – went extinct, dinosaurs had a chance to flourish in a way never before possible. By exiting the evolutionary stage, the phytosaurs and other Triassic losses made the reign of the dinosaurs possible. Molded into the rock, The Last Phytosaur is a silent witness to the close of one Mesozoic act and the beginning of another.

Reference:

Morales, M., Ash, S. 1993. The last phytosaurs? New Mexico Museum of Natural History and Science Bulletin. 3: 357-358